The present invention relates to discharge lamps, in particular to metal-halide discharge lamps.
FIG. 1 shows a conventional metal-halide discharge lamp. The lamp includes a ceramic discharge arc tube 5 with a capillary tube 10 extending from one side of the discharge tube 5. A feed-through 15 is inserted into the capillary tube 10 and sealed with a frit seal 20. The feed-through 15 includes four rod-like components; a tungsten electrode tip 25, a molybdenum coil 30, a cermet (50% Mo, 50% Al2O3) rod 35, and a niobium rod 40.
The tungsten electrode tip 25 extends into the volume of the discharge tube 5, to function as the discharge termination point. The molybdenum coil 30 is laser welded to the tungsten electrode 25 and extends into the capillary tube 10. The molybdenum coil 30 includes a molybdenum wire wound around a retained molybdenum mandrel. The particular geometry of the molybdenum coil 30 hinders the migration of salts from the fill gas in the discharge tube 5 into the capillary tube without causing excessive heat transfer up the capillary tube 10 from the discharge source.
The electrically conductive cermet rod 35 is laser welded to the other end of the molybdenum coil 30 to provide a material that is both resistant to the fill gases and salts as well as having a coefficient of thermal expansion similar to that of the wall of the capillary tube 10 and the frit seal 20.
Niobium rod 40 is laser welded to the other end of the cermet rod 35 and functions as a material interface between the interior and the exterior of the discharge tube 5 at the end of the capillary tube 10. Part of the niobium rod 40 sticks out of the end of the capillary tube 10.
The frit seal 20 is a solder glass material used to seal the niobium rod 40 to the capillary tube 10 so as to seal the interior of the discharge arc tube 5 from an outside atmosphere. The frit seal 20 extends from the end of the capillary tube 10 between the niobium rod 40 and the capillary tube and into the area where the cermet rod 35 is located. Since niobium is not resistant to the corrosive effects of the discharge tube fill, the frit seal 20 functions not only to seal the discharge tube from atmosphere but also to protect the niobium rod 40 from the discharge tube 5 fill. Niobium has the specific characteristic that its thermal expansion coefficient is very close to that of the alumina that forms the discharge tube 5 and the frit seal 20 to minimize seal cracks and leaks caused by the large temperature variations that can occur when sealing and operating the lamp. These materials each have a thermal coefficient of expansion of about 8xc3x9710xe2x88x926Kxe2x88x921. The cermet rod 35 would not be appropriate in the seal location even though it has appropriate thermal expansion characteristics because, unlike niobium, it can develop fissures that can spread and cause leakage of the seal.
The feed-through shown in FIG. 1 requires three laser welds with three different pairs of materialsxe2x80x94W to Mo, Mo to cermet, and cermet to Nb. The laser welds must provide intimate contact between the materials and provide good conductivity through the feed-through. The materials must be welded together to provide a straight feed-through that is easily slid into a capillary tube of a discharge lamp. The laser welds must be uniform and smooth so as to avoid burrs and the like that can inhibit passage of the feed-through into the capillary tube. The laser welds must be strong to avoid feed-through breakage during handling and shipping prior to being sealed in the discharge tube. Laser welding equipment is expensive and there are safety issues with its use. The laser set-ups are complicated requiring expensive fixturing with an inert atmosphere at the weld. Although the laser welding process is feasible, it is costly and complicated and one would rather not use laser welding.
In addition to the complications of laser welding, the feed-through contains four different materials, which have to be managed and understood from a material processing, lamp fabrication and lamp operating perspective. The cermet is expensive and its integrity is problematic at high temperatures relative to solid homogeneous refractory materials due to its potential to segregate into its base materials as well as develop fissures. Niobium absorbs hydrogen at low temperatures ( less than 100xc2x0 C.), oxidizes in air ( greater than 200xc2x0 C.) and readily absorbs hydrogen, oxygen and nitrogen at higher temperatures that causes it to be brittle and to change its thermal expansion characteristics. Further, the niobium is exposed outside the discharge tube, and thus the discharge tube must be used in an atmosphere with which niobium does not react. Niobium also restricts the atmosphere in which the feed-through can be cleaned before manufacturing the discharge tube. For example, high temperature (xcx9c1100xc2x0 C.) wet and dry hydrogen surface cleaning of the feed-through to rid surfaces of carbide and oxides impurities would be possible if not for the presence of the niobium. Additionally, unlike tungsten and molybdenum, niobium is not resistant to the corrosive effects of the discharge tube fill and has to be protected. The niobium puts further constraints on the lamp arrangement because the frit seal must cover the niobium and extend beyond the niobium/cermet weld, which in turn exposes the frit seal to higher temperatures.
In attempts to overcome the problems of the conventional discharge lamp, other high-intensity discharge lamps have been offered. U.S. Pat. No. 4,531,074 to Nagy et al., describes a feed-through for a 250 W high-pressure discharge lamp in which thin strands of molybdenum wire having a diameter of 0.05 mm, and preferably not more than 0.01 mm, are bundled together. The patent teaches that the diameter of the bundle should not exceed 0.15 mm in the case of molybdenum. The bundle is threaded through a bore in an aluminum oxide plug and connected to a tungsten electrode. The bore is sealed with melted vitreous enamel. The bundle is flexible to compensate for the heat expansion of the discharge tube.
U.S. Patent Publication 2002/0084754 to Allen et al. describes a feed-through for a low wattage ceramic metal halide (CMH) lamp with a niobium outer lead welded to an intermediate component comprising a molybdenum overwind on a Mo mandrel. The intermediate component is welded to an electrode comprising a tungsten shank with a W coil wound around one end of the shank. Allen et al. use reduced diameter mandrels with an increased overwind or use multiple overwinds to alleviate thermal expansion stresses that occur between the intermediate component and the ceramic lamp.
FIGS. 2a and 2b show another conventional discharge lamp such as that taught by U.S. Pat. No. 5,455,480 to Bastian et al. Specifically, FIGS. 2a and 2b describe a 100 W high-pressure discharge lamp 5a with a ceramic sealing element 21, an electrical feed-through 22 and a discharge vessel having cylindrical ends 6 through which the feed-through 22 extends. The feed-through 22 is made of alumina with metal wires threaded therethrough. The feed-through 22 is formed with at least two thin wires 23 having a diameter of about 0.25 mm. The wires 23 that extend into the interior of the discharge vessel are twisted together to form an electrode tip 25xe2x80x2. The wires in the cylindrical ends are either loosely bundled and surrounded by glass melt or individual wires 23 are fed through a plurality of bores in a ceramic plug and then surrounded by glass melt 29. The number of wires determines the current rating of the lamp. Bastian et al. also teach a lead wire connection extending outside of a capillary tube. The lead wire connection end of Bastian et al. is complicated requiring a niobium closing portion 28 and a niobium wound portion 27 in addition to a glass melt seal 29.
These lamps have problems specific to the particular design choice. The bores of both Nagy et al. and Bastian et al. must be sealed in addition to the seal required at the end of the capillary. In addition, the bores themselves must be formed in the plug (aluminum plug of Nagy et al. and ceramic plug in Bastian et al.). In Nagy et al., a bore must also be formed in the tungsten electrode. The wires of Nagy are also very thin having a maximum diameter of 0.01 mm for molybdenum. The lead wire connection end of Bastian et al. is difficult to produce requiring a niobium closing portion and a niobium spiraled portion in addition to the glass melt seal.
It is an object of the present invention to provide a feed-through and a discharge lamp free of the above-mention problems.
Specifically, an object of the present invention is to prevent seal cracks and leaking due to differences in thermal expansion coefficients of the components of the discharge lamp.
Another object is to provide a feed-through for a discharge lamp that has plural spaced apart feed-through wires that are sufficiently small so that an absolute magnitude of a difference between the thermal expansion of each individual wire and a seal of the lamp is sufficiently small for each wire so as to avoid cracks in the seal while a sufficient number of the wires is provided to meet the lamp""s power requirements.
Yet another object is to provide a discharge lamp that is simpler and cheaper to produce.
Still another object is to reduce the number of components of the feed-through.
Still yet another object is to reduce the amount of high precision laser welding that needs to be performed.
These and other objects are achieved by providing a feed-through having a ceramic core with a plurality of grooves along its outside length and wires in the grooves. The wires are twisted together at least at one end of the feed-through. The twisted wire may be used as the electrode inside the lamp or a separate electrode tip may be attached to the twisted wire bundle.